Process for heavy metal electrowinning

Abstract: Process for producing metals Me selected from zinc, nickel, cadmium and cobalt, characterized in that the corresponding water-soluble ammino complex Me(NH₃) n Cl m is formed, and such a complex, in an aqueous solution, is submitted to electrolysis in a cell separation means between the anodic and the cathodic compartments.

Description: It is known that, in general, in the electrolysis of aqueous solutions of chlorides, at the anode chlorine is developed, and the cathodic reaction can either be the development of hydrogen with production of alkalinity, or the precipitation of the metal, according to the position the latter occupies in the series of the electrochemical potentials, according to the following reactions: anodic reaction : Cl - - e → ½ Cl 2 cathodic reaction : Me + + e + H 2 O → MeOH + ½ H 2 or Me + + e → Me At acidic pH values, chlorine gas is developed. Under neutral or alkaline pH conditions, chlorine, owing to the increase in its water solubility, causes, by dismutation, the formation of hypochlorite and other oxygen-containing compounds, such as chlorate and perchlorate. In the case of alkali-metal chlorides at pH<4, chlorine is produced, and at higher pH value alkali-metal hypochlorites or, in the case of higher anodic potentials, alkali-metal chlorates and perchlorates are produced. Large amounts of chemical products are manufactured by this route. In the case of heavy metal chlorides (Cu, Co, Ni, Zn, Cd, Pb etc.), at a relatively acidic pH the metal is deposited at the cathode and chlorine is developed at the anode. The anodic compartment of the cell must be kept separated from the cathodic compartment by means of a diaphragm or a membrane, and said anodic comportament should be closed in order to make it possible for pure chlorine to be collected, first of all in order to prevent a so toxicant gas from getting dispersed in the environment, and, furthermore, in order to prevent chlorine from coming, by diffusion, into contact with the deposited metal, dissolving it. The split cell, the use of which is mandatory for these kind of processes, adds a considerable complication to the electrolysis facility and, in the event when an ionic membrane is used in order to separate the compartments, it also implies a very high equipment cost. The production of chlorine, parallel to metal production, constitutes another limitation to the application of the electrolysis of chlorides for producing metals, because it is necessary that the same process can make use of the chlorine it produces. This is the case, for example, in the Falconbridge process, which produces electrolytic nickel from aqueous solutions of chlorides and uses chlorine in order to oxidize the ore. In general, according to the prior art, the electrolysis of the aqueous solutions of heavy metal chlorides did not enjoy those important industrial applications which its potentialities would deserve thanks to the advantages it offers on energy side, due to the high conductivity of chloride solutions, as well as thanks to the anodic potential of chlorine development being lower than of oxygen development. The alternative solutions to the anodic chlorine development adopted heretofore are, e.g., the oxidation of Fe 2+ to Fe 3+ , or of Cu + to Cu 2+ which, by occurring at a lower potential than of chlorine development reaction, avoid the production of the latter, and offer an advantage as regards the cell voltage. An example is the Clear process, according to which in the cathodic compartment Cu is deposited, and at the anode iron and copper are oxidized: these, in their turn, are used in order to oxidize chalcopyrite, converting sulphide into elemental sulphur and dissolving copper. Another solution adopted is of using in the anodic compartment a solution of an oxyacid, e.g., sulphuric acid. In this case, in order to separate the anodic from cathodic compartment, an ionic membrane, and the anodic reaction turns into a water oxidation one: H 2 O - 2e → ½ O 2 + 2H + At the anode oxygen is developed and H + ions through the membrane, reach the cathodic compartment. DE 27 39 970 discloses a cell suitable for the electrowinning of heavy metals and actually used for the electrowinning of Zn, which has no separation means between the anode and cathode. FR 936 742 discloses a process for the electrowinning of Co from a solution containing Ni, whereby ammonia-containing complexes are formed in aqueous solution. EP 0 486 187 discloses a cell and a method for employing the same in the electrowinning of heavy metals (copper, nickel, cobalt and the like) from baths such as etchants, electroless plating baths and the like. Summarizing the present state of the art of metal electro winning from chloride solutions, one may state that, in the case of chlorine production, as well as in the case of alternative anodic reaction, a cell split by a diaphragm or an ionic membrane should be always used, with all of the facility complications and the higher costs involved by such a structure. The present invention aims at producing metal by electrolysis from aqueous solutions, overcoming the drawbacks displayed by the technology known from the prior art, which is reminded above. Such a purpose is achieved according to the present invention with a process according to claim 1. Beside the simplifications as regards the equipment and the easier facility operations, the process according to the present invention makes it possible the current efficiency values to be increased and the cell voltage to be reduced, and, consequently, a considerable reduction to be attained in energy consumptions per each unit of metal produced. These considerable advantages and improvements can be obtained according to the present invention as defined in the claims. To the solution containing the chloride of the metal to be produced, ammonia and ammonium chloride is added in order to form the ammino complex of Me(NH 3 ) n Cl m type, which prevents the metal hydroxide precipitation. The chloro-ammino complex is thus dissociated into [Me(NH 3 ) n ] m+ and mCl - . When the thus obtained solution is submitted to electrolysis, at the cathode the metal is deposited and ammonia is liberated from the complex, at the anode the chloride is oxidized to chlorine, but the resulting chlorine reacts in the vicinity of the same anode with the ammonia released and migrated from the anodic region, oxidizing it to nitrogen, according to the reaction: 3Cl 2 + 2NH 3 → N 2 + 6HCl or 3Cl 2 + 2NH 4 Cl → N 2 + 8HCl Thus, elemental nitrogen is developed instead of chlorine. Inasmuch as the reaction of oxidation of ammonia or ammonium ion to nitrogen displays a lower electrochemical potential than the oxidation potential of chlorides to chlorine, the anodic voltage stabilizes at a lower value than as observed in chloride electrolysis with chlorine gas development. The resulting reduction in the anodic voltage, added to the higher conductivity of chloride solutions, makes it possible the cell voltage to be decreased, with a decrease which may be as high as 30%, as compared to the known technique of electrolysis of metal sulfates in acidic solution. For the optimization of the voltage value, and in order to allow a high enough solubility of chloro-ammino complex to be achieved, the cell operating temperature should be higher than 40°C and lower than 80°C, and preferably is 60°C. The ammonia which is oxidized to elemental nitrogen must be replenished and the added amount is controlled by the pH value, which should remain constant around neutrality value. Another feature of the process is that, with the electrolysis occurring at pH values of round 7, the metal deposition takes place under much more competitive potential conditions than the alternative reaction of hydrogen development, with benefits as regards the current efficiency. The decreased cell voltage and the higher current efficiency contribute to reduce the energy consumption in metal winning. Another object of the present invention is a suitable facility for implementing the above defined process, which comprises a non-split electolytic cell, e.g., one in which the anode and the cathode are not provided with separation means, such as a diaphragm or a membrane means, between both cell compartments. In order to better disclose characteristics and advantages of the invention, an exemplifying, non-limitative embodiment thereof is reported in the following. Example : An amount of 500 g of technical zinc oxide with commercial purity was dissolved in 10 l of an aqueous solution with 250 g/l of NH 4 Cl, at the temperature of 60°C. At reaction end, with all oxide having been dissolved, 2.5 g of zinc powder was added in order to cement any impurities of Cu, Pb and Cd contained in a small amounts in the oxide. The purified solution was then circulated at 60°C inside a non-split electrolytic cell which contained a cathode consisting of a titanium plate between two insoluble anodes of graphite, wherein said solution was kept vigorously stirred by means of air blown under the cathode. By causing a current of 20 A to flow with an initial voltage of 2.7 V (2.85 V under steady-state conditions) during 10 hours, 229.6 g of pure zinc was deposited, with 40 g of NH 3 , added as a 129 g of aqueous solution at 31%, being consumed. The end solution had a pH value of 6.9 and contained 18.5 g/l of zinc in solution. When said solution was recycled, it was capable of leaching 225 g of zinc oxide. The cathodic current efficiency of the deposition was of 97.1%, and the energy consumption, limited to electrolysis, with power being supplied as direct current, was of 2.41 kWh/kg of zinc. The consumption of NH 3 , considered at 100%, was of 17.1% by weight, relatively to the weight of obtained zinc. As one may see from the above disclosure, taken into consideration together with the above reported example, the process according to the present invention makes it possible a full series of considerable advantages to be achieved as compared to the prior art, according to the purposes proposed hereinabove.

Claims: Process for electro winning metals Me selected from zinc, nickel, cadmium and cobalt, characterised in that the corresponding water-soluble ammino complex Me(NH 3 ) n Cl m is formed, and such a complex, in an aqueous solution, is submitted to electrolysis in a cell free from separation means between the anodic and the -cathodic compartments, wherein during said electrolysis at the cathode said metal Me is deposited with NH 3 being liberated, whereas at the anode chloride is oxidised to Cl 2 and the latter reacts with said ammonia liberated at the cathode and migrated to the anodic region, according to the reaction: 3Cl 2 + 2NH 3 → N 2 + 6HCl or: 3Cl 2 + 2NH 4 Cl → N 2 + 8HCl with N 2 being developed at the anode, said ammonia oxidised to nitrogen gas being restored in the electrolyte by controlling the pH to maintain it within the range 6-8. Process according to claim I, characterised in that said ammino complex Me(NH 3 ) n Cl m in aqueous solution is directly submitted to electrolysis. Process according to claim 1, characterised in that said ammino complex is formed by causing a suitable compound of said metal to react with ammonium hydroxide and ammonium chloride, and the so obtained ammino complex is submitted to said electrolysis.

Inventors: ZOPPI GIANNI, ZOPPI, GIANNI
Applicants: ECOCHEM AG, ECOCHEM AKTIENGESELLSCHAFT
Application year: 1994
Main classification: C25C1
IPC codes: C25C1